The present invention relates to a transport protocol for lossy links in communication networks. Specifically, this invention relates to improving transport protocol performance in networks having lossy links by using an erroneously received packet to trigger retransmission without invoking congestion compensation mechanisms.
Reliable transport protocols, such as the transmission control protocol (TCP), have been tuned for traditional networks comprising wired links and stationary hosts. These protocols assume congestion in the network to be the primary cause for packet losses and unusual delays. Congestion occurs when the requirements of the source(s) exceeds the transport capability of the network or the reception capability of the receiver. For example, where multiple senders transmit packets to a network switch faster than the switch""s buffer can forward the packets, congestion results and some received packets are lost by the switch.
Under the TCP protocol, an acknowledgment is usually, but not necessarily, transmitted for every packet. Because the TCP protocol is a byte-stream protocol, it also has the flexibility to send an acknowledgment for a sequence of bytes. The typical acknowledgment indicates the sequence number of the last consecutive packet successfully received; this type of acknowledgment is referred to as a cumulative acknowledgment. The acknowledgment is considered cumulative because it confirms that all messages up to the indicated packet have been properly received. Every time a receiver receives a group of packets, the receiver sends an acknowledgment identifying the last continuously complete sequence of received packets. For example, consider the case where one hundred packets are sent, but packets 59 and 61-100 are not received. When the receiver successfully receives packets 1-58, it will provide an acknowledgment that all packets up to packet 58 were received; when it successfully receives packet 60 but not packet 59, the receiver will again provide an acknowledgment that up to packet 58 was received. The second acknowledgment indicates that a packet was received out of sequence without receiving the next packet in sequence. Duplicate acknowledgments can indicate to the TCP protocol that a packet was lost. Most often the packet""s loss is due to congestion and some form of congestion compensation is necessary such as reducing the window size. Several schemes exist to retransmit packet(s) sequentially after recognizing that a packet was lost.
As an alternative to cumulative acknowledgments, acknowledgments can be provided which indicate which specific packets were received in error; these acknowledgments are known as selective acknowledgments (SACKs). A SACK can be embodied as a bit map, for example, where each bit of the SACK represents a packet status: xe2x80x9c1xe2x80x9d for a particular packet sequence number indicates the packet was received without error and xe2x80x9c0xe2x80x9d indicates the packet was received in error or was not received at all.
The TCP protocol avoids congestion by utilizing acknowledgments from the receiver and adjusting a sliding window for the sender. Rather than sending a packet and waiting for an acknowledgment from the receiver before sending another packet, the sender keeps track of the total number of unacknowledged packets sent and continues to transmit packets as long as the number of unacknowledged packets does not exceed a specified window size. The sender dynamically adjusts the window size by probing the communication network to determine the network""s capacity. As long as there is no loss, the window size is gradually increased. When a loss occurs, the window size is reduced and then slowly expanded. The sender can identify that a packet has been lost due to congestion either by the arrival of duplicate acknowledgments indicating a loss or by the absence of an acknowledgment being received within a timeout interval. This entire process of controlling the window size to limit congestion is known as flow control.
A number of compensation schemes can be used to reduce the window size upon detection of a congestion error and to gradually increase the window size back to the edge of error free operation. Such compensation schemes include the slow-start algorithm, fast recovery, and fast-retransmit. For example, under the slow-start algorithm, if the window size was one hundred packets when a congestion error occurred, the TCP protocol reduces the window size to one; the lost packet(s) is then retransmitted and the window size is expanded after each successful subsequent transmission by the number of packets last transmitted. In other words, the slow-start algorithm reduces the window size to one and then doubles the window size after each successful transmission as indicated by the reception of an acknowledgment (ACK).
When transmitted packets fail to be received by the sender for reasons other than congestion, however, congestion compensation measures, such as reducing the window size, result in an unnecessary reduction in end-to-end throughput and suboptimal performance. For example, wireless links are increasingly being used within a communication network. Transmission errors over wireless links are often due to reasons other than congestion, such as interference. Therefore, wireless links often suffer from sporadic high bit-error rates (BERs) and intermittent connectivity problems due to handoffs. Consequently, TCP performance in networks having wireless links suffers from significant throughput degradation and very high interactive delays due to the unnecessary use of congestion compensation mechanisms.
Several approaches have been suggested to avoid performance degradation over wireless links where noncongestion errors predominate. For example, A Comparison of Mechanisms for Improving TCP Performance Over Wireless Links, by Hari Balakrishnan, et al., ACM SIGCOMM ""96, Stanford, Calif., August 1996, discusses several. One such approach is to make the base station, which relays communication data from a source in the network to a mobile receiver, TCP aware. The base station keeps a copy of all packets forwarded to the mobile receiver until the base station is certain that the packets were received. If a packet is not received by the mobile receiver, then the mobile receiver sends to the base station the SACKs that are marked to indicate that a non-congestion related loss has occurred. Once the base station receives three duplicate marked SACKs, rather than automatically relaying these duplicate marked SACKs through the network to obtain retransmission from the source, the base station attempts to suppress the duplicate acknowledgment and retransmits a copy of the packet without invoking congestion compensation procedures. Because the base station retains copies of unacknowledged packets for multiple mobile receivers, the base station must retransmit the correct packet associated with the specific mobile receiver that failed to receive the originally transmitted packet.
The Balakrishnan scheme, however, has several shortcomings. First, in a lossy link where congestion is typically not a source of error, duplicate acknowledgments unnecessarily waste system resources and requires an unnecessary delay time until retransmission. In other words, because congestion is not the source of error and does not prohibit the first acknowledgment from being sufficient, anything more than a single acknowledgment unnecessarily taxes the system. Second, by making the base station TCP aware and requiring the base station to track the destination mobile receiver for each packet, significant buffering requirements at the base station are necessary. Furthermore, the base station must possess substantial processing capabilities to probe into the packet headers, and to classify and buffer packets according to TCP connections and process acknowledgments.
The present invention avoids sending duplicate acknowledgments and invoking a congestion mechanism when packets are received with bit errors due to the lossy link and not due to congestion. If congestion, however, is a source of error over links other than the wireless link, acknowledgments indicate that congestion is the source of error and that it would be appropriate for this system to invoke congestion mechanisms.
Additionally, the present invention is configured so that base station does not need to become TCP aware to improve TCP performance and avoid invoking congestion mechanisms when bit error is the source of errors. Thus, base station need not have the significant buffering requirements as is necessary in the prior art.
The present invention provides a transport protocol within a communication network for use by a receiver connected to the communication network by a lossy link. The receiver distinguishes between packets received with non-congestion bit errors and packets having been not at all received due to congestion.
When packets are received with non-congestion bit errors, packets are marked as having been received with a non-congestion error and then all received packets are passed to the software protocol for evaluation. The receiver sends selective acknowledgments indicating which packets were successfully received and which packets were received with non-congestion bit errors while suppressing duplicate acknowledgments to prevent the invocation of a congestion mechanism.
When packets are not received by the receiver due to congestion, acknowledgments are sent to indicate which packets were successfully received and which packets were not received at all. Duplicate acknowledgments can be sent to indicate congestion loss. Alternatively, an acknowledgment having a flag bit can be sent to indicate congestion loss.
Additionally, forward error correction bits can be added at a base station connected to the receiver over the lossy link. The added correction bits can be utilized for correct for any bit errors in the packet header to insure that a received packet has been properly delivered before any acknowledgments are constructed.